bug#29002: it.first_visible_x is erroneously 0 while horizontal scrolling.

[Eli Zaretskii]

> Date: Sun, 29 Oct 2017 20:29:16 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 29002@debbugs.gnu.org
> 
> > * A total of 5 screen lines.
> > 
> > * Line 1 is the current line.
> > 
> > * All 5 lines visually appear to be horizontally scrolled.
> > 
> > * Using the revised function bug-hscroll (below), we iterate over each 
> > screen line and generate the values for it.first_visible_x and it.vpos.
> > 
> > * it.first_visible_x returns as being 0 for each of the 5 screen lines.
> 
> This cannot be true: if every line in the window is hscrolled,
> first_visible_x must be non-zero, right after the call to
> start_display.

Forgot to say: I cannot reproduce your results.  In my case,
first_visible_x is non-zero all the way through the loop, as expected.

Are you sure this isn't a side effect of one of your other changes?